The glacial streams of the McMurdo Dry Valleys have extensive cyanobacterial mats that are a probable source of fixed C and N to the Valleys. The research will examine the interplay between the microbial mats in the ephemeral glacial streams and the microbiota of the hyporheic soils (wetted soil zone) underlying and adjacent to those mats. It is hypothesized that the mats are important sources of organic carbon and fixed nitrogen for the soil communities of the hyporheic zone, and release dissolved organic carbon (DOC) and nitrogen (DON) that serves the entire Dry Valley ecosystem. Field efforts will entail both observational and experimental components. Direct comparisons will be made between the mats and microbial populations underlying naturally rehydrated and desiccated mat areas, and between mat areas in the melt streams of the Adams and Miers Glaciers in Miers Valley. Both physiological and phylogenetic indices of the soil microbiota will be examined. Observations will include estimates of rates of mat carbon and nitrogen fixation, soil respiration and leucine and thymidine uptake (as measures of protein & DNA synthesis, respectively) by soil bacteria, bacterial densities and their molecular ecology. Experimental manipulations will include experimental re-wetting of soils and observations of the time course of response of the microbial community. The research will integrate modern molecular genetic approaches (ARISA-DNA fingerprinting and ultra deep 16S rDNA microbial phylogenetic analysis) with geochemistry to study the diversity, ecology, and function of microbial communities that thrive in these extreme environments. The broader impacts of the project include research and educational opportunities for graduate students and a postdoctoral associate. The P.I.s will involve undergraduates as work-study students and in REU programs, and will participate in educational and outreach programs.
The McMurdo Dry Valleys of Antarctica are the coldest, driest environments on earth and the largest ice free region on the continent. Stretching over about 6500 km2, the Dry Valleys experience minimal precipitation, strong katabatic winds, and temperatures below -60 °C in austral winter. These extreme conditions are the reason that the Dry Valleys were once considered to be unfavorable for life; in fact they host a microbial community consisting of phylogenetically diverse microorganisms including bacteria, microalgae and fungi. During the austral summer, darkness turns to light, temperatures warm, and the soils adjacent to lakes, ponds, and ephemeral glacial meltwater streams become wetted. These wetted zones support extensive microbial mats containing N2 fixing cyanobacteria, however, studies that quantify physiological rates and elemental cycling associated with these mats, as well as the abundance, population structure and activity of associated heterotrophic microbes, are limited. In such extremely oligotrophic soil systems these mats are likely to be a source of organic matter and essential nitrogenous nutrients to the entire Dry Valley system by the transportation of materials formed in the mats through groundwater flow or redistribution by the wind. We undertook studies to establish the importance of these microbial mats as model primitive ecosystems living at the extreme of conditions on earth with respect to temperature, and light, water and nutrient availability. They are metabolically active during the brief austral summer when temperatures briefly rise above freezing and meltwaters are available. However, virtually nothing is known of their biogeochemical impact in these environments or on the microbial communities associated with them. Our data demonstrate that these mats attain high rates of C & N fixation which in turn substantially elevate dissolved organic carbon and inorganic N pools and thereby promote enhanced microbial secondary production. Moreover, the densities of the microbial assemblages in the hyporheic (wetted soil) zones associated with these mats were greatly elevated and their phylogenetic composition distinct relative to dry soils. Components of the flora which live on organic matter may also contribute to nitrogen inputs through N2 fixation. Indeed, both molecular and preliminary activity-based results indicate that in places a large diversity of non-phototrophic bacteria are the dominant organisms containing nitrogenase genes in wetted microbial mats of these soils. This sub-population can account for over 50% of the N2 fixation activity in some mats. These surprising results are shifting the long-held paradigm that cyanobacteria are the primary N2 fixers in Dry Valley soils. However, focused research is now needed to truly understand the importance and drivers of non-photosynthetic N2 fixation across the entire Dry Valleys system, as each valley has its own unique characteristics.
